This might happen if you are using Implicit Styles with XAML binaries. If you are using Implicit Styles please ensure that the NoXaml binaries distribution is registered in the Visual Studio Toolbox. You can do that with the help of the Toolbox Configurator. Open it and select the NoXAML entry with the desired version. After the Toolbox Configurator restarts the Visual Studio try again to add a control from the Toolbox.
Also please check whether the version of telerik binaries in your application is the same.
More information about how you have added the binaries would be of great benefit in the process of resolving your issue.
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